by John Gatski In nearly 20 years of reviewing Pass Labs amplifiers and using various models as reference, I have followed the evolution of the company’s essential MOSFET designs. From the Aleph to the original Class A/AB X Series, to the highly regarded Class A XA models, the amps have steadily improved to attain Pass’ ultimate “musical” amplifier goal. The platinum priced Xs Series is the culmination of that evolution.Built to order, the $65,000 Xs-150 monoblock reviewed here and its big brother, the $85,000 Xs-300, are for well-off audiophiles who want the ultimate hi-fi amplifier — in terms of build quality, as well as delivery of a luscious audio sound stage that is the epitome of width and depth.

Features

The Xs-150 is a 150 wpc channel, high-power, pure Class A amplifier that is built into two chassis-per-channel; one box houses the amp section and the other the power supply. They are connected by a twist-on connector, heavy duty power umbilical cord.

At a premium price, the Xs-150 is equipped with high-end parts in the signal path, power supply, capacitors, etc. and includes such niceties as heavy duty speaker binding posts with solid-metal knobs. premium XLR and RCA connectors, and beefy IEC receptacle and power switch. The familiar, blue-hue, illuminated Pass analog current meter adds that bit of extra class. Normal Class A operation leaves the needle squarely in the middle.

I have listened to a lot of amps in my 25 years of reviewing, but the ML Montis electrostatics/Pass Xs-150 pairing delivered premium spaciousness in transients, reverb decay and instrument subtleties that make music reproduction sound like live music.

The two-chassis-per-channel equals a hefty weight to manage in set up: 100 pounds for the amp and 130 pounds for the power supply. Even with rear-mounted, rack handles, you will need two people if you are setting up in a rack arrangement. I was able to unbox and maneuver the components for floor placement, but it took care, deliberate handling. I would not advise it if you are not used to moving 100+ pounds.

Since 2007, Pass Labs has produced its Class A, XA.5 amps, using the patented Supersymmetry MOSFET design. But, according to Pass, the evolutionary improvements, based on previous designs, along with newer techniques, has increased the musical conveyance of the XS amp.

According to Nelson Pass, the major design differences between the XA .5 and Xs are:

•Separate power supply chassis for lower electromagnetic noise:

•Upgraded power supplies with greatly enhanced storage capacitance;

•Banks of redundantly parallel high speed/soft recovery rectifiers;

•Improved high frequency noise filters;

•Larger (and quieter) transformers;

•Improved passive decoupling;

•Lower power standby;

•200 k-ohm input impedance, balanced with negligible capacitance.In asking Pass about the genesis of the Xs amplifier, he said that it is more evolutionary than revolutionary. “In 2008, we began work on the Xs amplifiers and ultimately drew upon a design features of the Aleph 0 from 1992,” Pass explained. “Like the Xs, it used a complementary follower output stage in parallel with a very large constant current source — enough to provide single-ended Class A bias to rated power.” He continued:”The simplified topology of the Xs amplifier looks like that of the ".5" amplifiers — except that the hardware is much bigger, and the bias currents involved are more massive. Also, there are some other details — involving new methods of applying local and global feedback and more sophisticated regulation of the voltages and currents.” Pass said that the sound of the best tube amplifiers has influenced the design of the Xs series, a characteristic I very much noticed while doing this review. “The working out of the details of the Xs amplifier depended very much on the information about the subjective performance we were obtaining with our experiments with custom SIT (aka VFET) transistors rendered in Silicon Carbide by SemiSouth. These parts captured some of those very elusive qualities belonging to the finest tube designs and informed the development of the Xs design, a process that took about four years.”

Rear panel connections: XLR, RCA and 12V trigger

Much of the research and implementation of the Xs design also has been incorporated into Pass’ new XA.8 series of Class A amps, though with a bit more modest specs, Pass added. “The XA .8 series is very much the same topology as the Xs, but the hardware and bias currents are scaled down to reasonable values. The single-ended bias currents are much larger than those of the .5 series, but not enough to support single-ended bias to full power. It is reasonable to think of them as more modest versions of the Xs amplifiers — they are smaller — but they still make use of all the circuit details and parts that went into the Xs.” Spec-wise, the Xs-150 delivers 150 watts from each amp. Input impedance is 200 Kohm balanced; 100 Kohm single-ended. Gain, 26 dB; Bandwidth, DC to 150 kHz (-3 dB); and 30 amps peak output current. The distortion is rated at .03 percent at 1 watt and 1% at full power. Power consumption is 500 watts (with a lot of heat dissipation!). The main amp chassis and power supply each measure 19 W x 11.5 H x 21.5 D. The power supply section weighs 130 pounds; the amp chassis is 100 pounds. To conserve space, it is recommended to stack the amp chassis on top of the power supply chassis.The setup It took me about an hour to wrestle the shipping boxes, unpack the amps and position them into my downstairs listening room. Although it is logical to use a partner to unpack and place these amps, I did it solo. Sometimes, you just can’t wait for a helper; I even managed to “power lift” the amps on top of the power supplies.

Big Class A power means big supply capacitors

Associated equipment for the review included an Oppo BDP-105 universal player, Oppo HA-1Class A output DAC/headphone/line-out preamp, Benchmark DAC2-D DAC, Mytek Stereo192-DSD DAC, Coda and Pass XP-10 preamps. Other sources included Dell tablet with USB Audio Player Pro (up to 24-bit/384), Clear Audio turntable, and Rogue Audio Model 99 Magnum tube line preamp with tube phono stage. Speakers included my reference MartinLogan Montis electrostatics, Westlake LC 8.1’s, Legacy Studio’s and Pass SR-2 three-ways. In for review, I also mated the Xs-150 with the new Benchmark SMS-1 compact speakers (review upcoming), Bryston Mini-T (review upcoming) and the recently reviewed Legacy Expression tower. All cables were courtesy of Wireworld; AC cables and power distributor were provided by Essential Sound Products Essence Reference II. I did a lot of listening using the Oppo HA-1 discrete headphone amp output, driving the amplifiers through a 1/4-inch-to-RCA adapter and a pair of premium Wireworld unbalanced cables, but I also used other preamps as well. Though the factory nominally burned in the amps, I gave them an extra day of break in, uninterrupted. And the word burn has more significance when breaking in a Class A amp that idles at 500 watts. It takes about 90 minutes for the amps to get to full temperature; about 110 degrees on the chassis. And that heat in a closed room in Summer easily pushed the temperature into the mid ‘90s — even with the AC on.

Inside the amp chassis: more big caps, lots of MOSFETS

In listening sessions, I could only handle the heat up to four hours before I turned it off for a break (and a shower). Bigger rooms should have less heat build up, and normal-use amp heating will vary with room size, ventilation and your willingness to nudge down that thermostat. Although I did most of my listening during warm weather, it is the perfect cold weather hi-fi amplifier.The audition After a couple days of the, er, burn in, I was ready to listen. First up were my reference MartinLogan Montis, the advanced electrostatic panel-design loudspeaker with powered woofer from 300 Hz down. These speakers have the ability to present maximum musical space, in terms of depth and width, as transmitted by the upstream components. If the sources, preamp and amp can deliver the sonic portrait, these speakers pass it on. A good sonic test for any amplifier. From the very first play of “Britta’s Blue” cut from the Anthony Wilson — Our Gang SACD, the separation of the Gibson guitar, drums and Hammond organ was astonishing through the Xs-150; I could almost walk between the instruments. And the extra sonic dimension just above the typical listening sphere, thanks to the Montis extra height gets more coverage as well. I have listened to a lot of amps in my 25 years of reviewing, but the Montis/Pass combo delivered premium spaciousness in transients, reverb decay and instrument subtleties that make music reproduction sound like live music. The general character of the Xs is definitely Class A; there is a warmth and smoothness that is very much like premium tube amp, yet the transients are taut and bass is focused. Versus an older first generation XA30.5 Class A amp that I had on hand, now priced at $6,000, the smaller, older amp is not in the same league as the Xs-150. The Xs' enveloping sound stage is much more live by comparison. The first gen X350.5 (current version is $11,500) is closer in sound stage, but still not quite there, plus does not have that musical richness on Classical and Jazz.

The review pair od Pass XS-150s

On the Janos Starker — The Bach Cello Suites — the early 1960s Mercury Living Presence recording is a minimalist originally recorded with three tracks, but mixed to stereo. With all this musical “space” captured intact via DSD transfer, the Xs-150s showcase the cello’s sonic panorama. And the amp is so detailed that Mr. Starker’s breathing, bow handling noise, chair squeaks, etc., all could be more clearly heard using this set of amps. You can hear those subtle sounds with other amps, but they are more noticeable with the Pass Xs. And along with that audible delineation, the tone is rich and pure. The cello is big, not overly bloomy. The string harmonics are all there, and the aural picture is bigger via the Xs — versus other amps. The ability to deliver this open, musical sound stage with all the subtle detail, is the amp’s forte. Classical solos, duos, trios, quartets, Classical guitarists, Jazz guitar soloists, small jazz combos are like live performances in the living room! Really good hi-res transfers and newer hi-res recordings heighten the experience On the Steve Davis — Thought About You, a Tom Jung-recorded Jazz SACD from the late 1990s, that spacious, warm, yet dynamic tone — with percussion, piano and guitar — is world class through the Xs. The recording has a wide, instrument-distinct presentation with crisp, precise percussion and piano notes; the Pass takes it up another level with its ultra spaciousness. In keeping with the “minimalist recordings sound the best theme,” I played an “ultra-res” stereo 24-bit/384 acoustic guitar track that I made, using a new Taylor 810E guitar, recorded with two Audix SCX-25 condenser microphones through a True P2 discrete microphone preamp that fed an Antelope Eclipse A/D and Macbook Pro. The recording has intricately strummed and picked layers with excellent width and dynamics in ultra high res.

Again the Pass Xs-150 transports these seemingly simple recordings into life-like performances. It does not sound exaggerated. In fact, it is super natural in tone and presentation. The amp finds the track’s essential bigness in the stereo image and passes it on.

Again the Pass Xs-150 transports these seemingly simple recordings into life-like performances. It does not sound exaggerated. In fact, it is super natural in tone and presentation. The amp finds the track’s essential bigness in the stereo image and passes it on. My observation of the Pass’ character did not waver when using other speakers. All speakers may not have the space impression of the ML electrostatics, but the results were just as impressive within the character of the particular speaker. Through Pass’ own SR-2, three-way tower that I reviewed in 2013, the Xs-150’s spaciousness and solid bass performance on George Benson — Breezin’ 24/96 DVD-A was quite a treat for my ears. The piano solo part on “Down Here On the Ground” emerged from the mix with the just amount of upper register tinkle. I also had a pair of Benchmark’s new SMS-1 bookshelf speakers for a brief couple of sessions with the Xs-150s, as well as the new Bryston Mini-sT (review upcoming). The Pass’ Xs-150‘s ability to project the image brings out the best in smaller speakers. You might think that these $65,000 amplifiers would need a big, premium, $10,000+ speakers to achieve their best. But even with small, under-$3,000, audiophile speakers, the amps shined their sonic spotlight. Piano tone really stood out with the Pass Xs amps and the new Benchmark speakers. (Stay tuned for that review) I though the Pass might be more of classical/jazz/acoustic music amplifier, which are often less processed. But listening sessions with numerous, well-recorded, less processed Pop recordings pleasantly surprised me. The 24/96 remaster of Led Zeppelin’s “Stairway To Heaven,” from the reissue of the Led Zeppelin IV HD Tracks download, really benefits from the Xs' wide sound stage prowess. The separation among the acoustic guitar and electric rhythm guitar layers interjects a breath of openness to this played-to-death classic.

There are always going to be the extravagant products, but it is hard to make a quantifiable justification over a lesser priced comparator because high-ticket items are purchased for more than sheer performance.

The Dire Straits — Brother in Arms DVD-A reissue also sounded quite good through the Pass. The title track, again, really comes alive. The percussion, guitar/vocal/percussion/accordion interplay gets that full spread across the image. And I was knocked out by how good the 24-bit/192 remaster of The Commodores' hit "Sail On" sounded; the song came from the 1979 Midnight Magic album, which is now available from HD Tracks as a 24/192 download. The remaster brings out all the multitrack embellishments of "Sail On," which are easily heard through the Xs-150’s big-ticket presentation. Subtle steel guitar licks, acoustic and lead electric guitar fills, funky bass line precision, shaker percussion really pop from this hi-res transferred track. I could hear these various instruments via the Pass and the Montis as well as I could through an audiophile DAC and headphones. Wow! As with most amps, heavily compressed Pop and Rock gets a bit mushy through the Xs-15; that bit of Class A warmness doesn’t flatter those types of recordings as much. Cleaner, less complicated Rock music work better, at least with the speakers I was using. I think the Pass X.5 series like my X350.5, which operates more in Class A/B at louder levels is a amplifier for the dense stuff. The Xs-150 tandem has no real sonic flaws, or any ergonomic negatives as well. Besides its astronomical price, the biggest negative is the heat. The inefficiency of Class A means lots of heat from two, very powerful Class A amps housed in four boxes. Even lowering the thermostat on the AC, located on another floor, I could not keep the listening room cool enough beyond 4 hours. Maybe if I had the rest of the house at 65 degrees I could have kept it under 85 degrees. Aah, the afterglow of Class A.

“The XA .8 series is very much the same topology as the Xs, but the hardware and bias currents are scaled down to reasonable values...It is reasonable to think of them as more modest versions of the Xs amplifiers — they are smaller — but they still make use of all the circuit details and parts that went into the Xs."